1. Field of the Invention
The invention relates to a process for the preparation of D,L-methionine or a salt of D,L-methionine, starting from the components 3-methylmercaptopropionaldehyde, hydrogen cyanide, ammonia and carbon dioxide or 5-(2-methylmercaptoethyl)-hydantoin or a salt of methionine or those components from which the above-mentioned components can be prepared, optionally in the presence of water.
The synthesis steps can be illustrated by the following reaction equations:
5-(2-methylmercaptoethyl)-hydantoin formation: ##STR1##
M stands for alkali metal, alkaline earth metal, ammonium, particularly potassium.
Advantageously, the process steps of 5-(2-methylmercaptoethyl)-hydantoin formation, methioninate formation and methionine liberation may be carried out continuously, advantageously in successive stages which are integrated in a process which, as a whole, takes place continuously, in particular.
Particularly advantageously, the components ammonia and carbon dioxide are recycled in accordance with the process possibilities, i.e. used again in the preceding process stages. Particularly when potassium is used, all the agents containing alkali metal are recycled to the process, if possible.
2. Background Information
The formation of 5-(2-methylmercaptoethyl)-hydantoin is known in principle. Generally speaking, the starting materials are either the components described under 1) or components from which these components can be prepared. These are, in particular, alkali metal- or ammonium salts in the case of the components hydrogen cyanide, ammonia and carbon dioxide, and acrolein and methyl mercaptan in the case of the 3-methylmercaptopropionaldehyde component, as described further below. Chem. Rev. 46 (1959) 422-425 describes the preparation of substituted hydantoins by reaction of the corresponding aldehydes and ketones with alkali metal cyanides and ammonium carbonate. The reaction is carried out either using stoichiometric proportions of the substances at 80.degree. C. and 3 bar or using several times the stoichiometric proportion of ammonia at temperatures up to 60.degree. C. and at normal pressure (DT-PS 11 66 201). It is also known to prepare 5-(2-methylmercaptoethyl)-hydantoin from 3-methylmercaptopropionaldeyde, ammonium carbonate and cyanides. The reaction is carried out initially at 40-120.degree. C., the reaction mixture is then adjusted to a pH below 4 and the reaction is ended at 50-100.degree. C. (US-PS 2 557 913). Moreover, it is known to prepare 5-(2-methylmercaptoethyl)-hydantoin by providing a solution which was prepared by dissolving 3-methylmercapto-propionaldehyde in an aqueous solution of ammonia, carbon dioxide and hydrocyanic acid or the salts thereof and in which, optionally, a reaction to hydantoin has taken place partially or wholly, and introducing into this solution an aqueous solution of ammonia, carbon dioxide and hydrocyanic acid or the salts thereof and, separately, the 3-methylmercaptopropionaldehyde, and carrying out the reaction by heating the mixture to a temperature up to 100.degree. C. at normal pressure (DT-OS 16 20 332). In Japanese patent JP 48-005763, 3-methylmercaptopropionaldehyde is reacted with hydrogen cyanide or the salts thereof and ammonium carbonate in the presence of ammonia and at 80.degree. C. in 1.5 hours to form 5-(2-methylmercaptoethyl)-hydantoin in a 98.5% yield. Added metal ion complexing agents in the presence of water lead to a 97.8% yield (JP 48-004465). A similar reaction in the presence of organic solvents at 50-200.degree. C. under pressure in the liquid phase is described in patent JP 40-36676. A one-pot variant starting from acrolein, methylmercaptan, hydrogen cyanide and ammonium carbonate in water at 50-70.degree. C. leads within 2 hours to a hydantoin which is saponified to D,L-methionine (JP 50-004018).
The reaction described in JP 52-027768 takes place in a similar manner, but with the addition of amino acids such as methionine, threonine, glycine, alanine, or leucine. 3-Methylmercaptopropionaldehyde, carbon dioxide, ammonia, hydrogen cyanide and caustic alkali at 80.degree. C. lead to 97% 5-(2-methylmercaptoethyl)-hydantoin within 2 hours (JP 50-018467).
3-Methylmercaptopropionaldehyde, sodium cyanide and ammonium carbonate in water in the presence of potassium thiosulphate or potassium carbonate give 5-(2-methylmercaptoethyl)-hydantoin (SU 740770). In the single-stage reaction of acrolein with methyl mercaptan, hydrogen cyanide and ammonium carbonate, 5-(2-methylmercaptoethyl)-hydantoin is produced in an 85% yield (Asahi Chem. Ind., Agric. Biol. Chem. 52, 589 (1988). The Chinese patent CN 85 1085905 also describes a single-stage reaction, but with the addition of methionine in acetic acid with 91% conversion to 5-(2-methylmercaptoethyl)-hydantoin.
A 5-(2-methylmercaptoethyl)-hydantoin prepared by the known methods is contaminated to a considerable extent by 5-(2-methylmercaptoethyl)-hydantoic acid, 5-(2-methylmercaptoethyl)-hydantoic acid amide, methioninamide, methionine nitrile and methylmercaptopropionaldehyde cyanohydrin, iminonitrile and polymers. Whilst the first three compounds mentioned, like the hydantoin, are converted to methionine during alkaline hydrolysis, the other compounds or the saponification products thereof enter the saponification solution and the methionine to be isolated later, where they can be separated only with great difficulty. This is particularly so when the methionine is prepared from the hydantoin and separated from the reaction mixture using carbon dioxide and the mother liquor recycled. The methionine obtained is discoloured and has poor storage stability.
Alkaline hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin is by no means new. U.S. Pat. No. 2,527,366 and U.S. Pat. No. 2,557,913 describe the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin in an aqueous barium hydroxide solution under pressure and at elevated temperature. These processes require substantial quantities of expensive barium hydroxide, however; moreover, the barium has to be separated again as a neutral salt.
It is known from U.S. Pat. No. 2,557,920 that .alpha.-amino acids are produced by saponification of hydantoins using sodium hydroxide. In these processes, however, at least 3 moles of sodium hydroxide per mole of hydantoin are required. The situation is similar when potassium hydroxide is used.
Moreover, it is known from U.S. Pat. No. 4,272,631 that a mixture of alkali metal and alkaline earth metal hydroxide may be used to saponify 5-(2-methylmercaptoethyl)-hydantoin. In these processes, however, the alkaline earth metal ions first have to be separated during the liberation of methionine, so maximum yields of only 80.5% are obtained.
In U.S. Pat. No. 4,259,925, the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin is carried out under pressure at 105 to 230.degree. C. in a medium that contains a metal hydroxide and an alcohol with a boiling point of 125 to 130.degree. C. A disadvantage is that the high-boiling alcohol has to be recovered. Moreover, the yield is only 65%.
The hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin using an aqueous solution of alkali carbonate and/or alkali hydrogen carbonate is described in DE-PS 19 06 405. Ammonia and carbon dioxide are removed continuously during hydrolysis. The preferred alkali metal carbonate is potassium carbonate; a molar ratio of hydantoin to alkali metal of 1:1 to 1:5 is used. Hydrolysis is carried out under pressure at 120 to 220.degree. C. The continuous pressure apparatus is composed of three rotary evaporators set up at great expense in series. The alkali metal methioninate solution is used to liberate D,L-methionine with carbon dioxide; the mother liquor obtained from the separation of the methionine that crystallises out is recycled, optionally with the removal of 1-2%, and used again for the hydrolysis of hydantoin.
DE-AS 15 18 339 describes a process in which gaseous reaction products (ammonia and carbon dioxide) produced during hydrolysis are removed from the reaction in order to shift the reaction equilibrium towards the amino acid, as a result of which the yield is increased. In order to achieve this, however, a complex arrangement of apparatus is required to regulate the pressure of the gas streams.
Japanese patent 49/116 008 describes a process in which the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin is carried out in the presence of vanadic acids, molybdic acids, tungstic acids or the derivatives thereof. The yields are about 70%. The separation of the catalyst is difficult. In order to prepare a highly concentrated methionine-containing solution, alkali metal e.g. a potassium compound, must be added.
It is known from the Japanese patent application 75/106 901 (C.A. 84,44666k (1976)) that methionine is prepared by hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin in the presence of about 1.2 equ. of sodium hydroxide and about 9 equ. of ammonia at 180.degree. C. The sodium methioninate solution obtained as an intermediate with this mode of operation necessarily contains not only sodium methioninate, however, but also sodium carbonate which precipitates whilst this reaction is being carried out and is therefore troublesome, particularly in a continuous process. The same is true of the use of potassium hydroxide and of the method of operating according to DE-PS 19 06 405.
DE 26 14 411 A describes the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin with water in the presence of imidazole at 160.degree. C. The yields are low and here, too, an alkali metal compound has to be added in order to obtain a high solution concentration.
Japanese patent applications JP 03/95145 and JP 03/95146 describe the hydrolysis of hydantoins with water at elevated temperature and elevated pressure in the presence of metal oxides or oxide mixtures, for example ZrO.sub.2, TiO.sub.2, Nb.sub.2 O.sub.5 or TiO.sub.2 --Nb.sub.2 O.sub.5 ; the yields are only 65 to 66%, however. These solutions must be neutralised with an alkali compound.
All these processes bring either low yields or have the disadvantage that methionine or salts such as carbonates are precipitated during the process, as a result of which further process steps are brought about and industrial-scale processes in particular, more particularly continuous processes, are scarcely possible.
The liberation of methionine from the alkali metal salt is generally known. According to the principle that strong acids liberate weaker acids from their salts, free D,L-methionine is precipitated e.g. with hydrochloric acid, sulphuric acid, phosphoric acid or a strongly acid ion exchanger (DE 21 40 506 C; DE 21 22 491 C; DE 29 12 066 A; BE 877 200, U.S. Pat. No. 3,433,832, FR 1 532 723). The alkali metal salt obtained as a by-product must then, however, be separated. As the acid used is not generally recovered, this method of operating is unsuitable for a continuous economic and environmentally acceptable production process.
As described for example in the patents U.S. Pat. No. 2,557,913, DE-PS 19 06 405 and JP 42/44056, D,L-methionine was therefore precipitated advantageously from the hydrolysis solutions of 5-(2-methylmercaptoethyl)-hydantoin with carbon dioxide in aqueous solution. In this method, D,L-methionine is usually obtained in the form of thin leaves or as flakes. This causes the product to be difficult to filter and prevents the crystal cake being washed out; moreover, a D,L-methionine with poor flow properties and a tendency to form lumps is obtained. In order to counter these disadvantages, according to Japanese patent JP 42/44056, additives such as casein or water-soluble, high molecular weight-cellulose derivatives are added during the precipitation of methionine with carbon dioxide.